Superficial unfractured dry crystalline bedrock (e.g. granite) can constitute the diffusive hot storage for a large-scale thermoelectric energy storage by thermal doublet, an ice storage being the latent cold sink of the thermal doublet and supercritical CO2 (sCO2) the heat transfer working fluid circulating inside closed-loop vertical geothermal exchangers. Operative 30°C–140°C thermal range of the diffusive hot storage will not alter mechanical resistance or thermal conductivity of the encasing bedrock. Technological issue for thermal coupling of the geothermal exchanger with the bedrock at working fluid temperatures above 100°C is solved by coaxial exchanger design implementing silicone rubber as wall material. Difficulties of modeling heat transfer for the full-scale geothermal exchanger due to Reynolds number up to 106 for the flow regime inside the exchanger are addressed through a simplified modeling approach. Experimental investigation on 1/10 scale heat exchanger prototype with sCO2 as working fluid is conducted to study heat transfer performance and storage dynamics, and also to validate the full-scale modeling.

1. Introduction

Massive integration of intermittent renewable energy production generates new challenges for the supervision and regulation of electric grids. The use of flexible but carbon-intensive technologies such as gas turbines has been the main solution in order to ensure the balance between demand and supply, maintaining grid frequency and power quality. Large-scale electricity storage is an alternative with lower environmental impact. Pumped-Storage Hydroelectricity (PSH) accounts for more than 99% of the worldwide electricity storage bulk capacity, representing around 140 GW over 380 locations, and covers a power range varying from a few hundred of megawatts to a few gigawatts [1]. Despite having a long lifetime and being the most cost-effective energy storage technology, PSH requires construction of large water reservoirs, leading to high environmental impact. In addition, most suitable locations have already been used in developed countries. Compressed-Air Energy Storage (CAES) is at an advanced stage of development but accounts only two existing power plants because of an exceptional underground geological set-up needed for multi-megawatt energy storage. Thermo-electric energy storage (TEES) is an alternative that could provide large-scale electricitet al.y storage. Principle of TEES is as follows: during periods of excess electricity generation, a vapor compression heat pump consumes electricity and transfers heat between a low-temperature heat source and a higher temperature heat sink. The temperature difference between the heat sink and the heat source can be maintained for several hours, until a power cycle is used to discharge the system and generate electricity back to the grid during peak consumption. Mercangöz et al. [2] showed that the first study on TEES dates back to the 1920s, and described the general concept of this technology based on two-way conversion of electricity to and from heat. The authors analyzed a TEES system with CO2 transcritical cycles, hot water and ice tanks as respectively hot and cold storage reservoirs. The ABB Corporate Research Center [3] described a way to store electricity using two hot water tanks, an ice tank and CO2 transcritical cycles. Desrues et al. [4] studied a high temperature TEES system involving argon as working fluid following a closed thermodynamic Brayton cycle.

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